In a disk player for reading information recorded in an information disk (hereinafter referred to simply as a disk), such as a compact disk (CD) or a video disk, the rotation of the disk is controlled so that the linear velocity of a light beam irradiating tracks on the disk relative to the disk is constant, on the basis of rotational information obtained from a reproduction signal from the disk. This control is referred to as a constant linear velocity (CLV) control. In the CLV control, recorded information is read out with an optical pick-up that is movable along the radial direction of the disk. On the other hand, for a disk recording or reproducing computer data, the rotation of the disk is controlled so that the rotational speed is equivalent to a constant angular velocity. This control is referred to as a constant angular velocity (CAV) control.
When control of disk rotation is started, i.e., when disk rotation is started, the rotation of the disk is accelerated until it reaches a prescribed rotational speed. When the rotational speed is reached, the above-mentioned CLV control or CAV control is performed. On the other hand, to stop the rotation of the disk, the rotation is decelerated, i.e., the rotational speed of the disk is decreased.
Further, the disk player is equipped with a focus servo system (hereinafter referred to simply as a focus servo) for controlling the optical pick-up so that a light beam for detecting information, emitted from the optical pick-up, is accurately focused on a recording surface of the disk, and a tracking servo system (hereinafter referred to simply as a tracking servo) for controlling the optical pick-up so that the beam spot accurately follows recording tracks on the disk.
A description is given of a conventional disk rotation control apparatus. When the apparatus is started, a disk is appropriately rotated, and a focus servo and a tracking servo are operated. In this state, when the rotational speed of the disk according to the position of a pick-up relative to the disk is appropriate, a reproduction signal (RF) synchronizes with a reference clock, and a PLL is locked.
FIG. 22(a) shows a fundamental structure of a PLL. In FIG. 22(a), an edge of a reproduction signal (a cross point of an eye pattern) is applied to the PLL as an input A, and a continuous oscillation waveform from a voltage controlled oscillator (VCO) is applied to the PLL as an input B. FIG. 22(b) shows clock pulses applied to the PLL. It is assumed that the disk rotates at a speed lower than a target rotational speed for the position of the optical head relative to the disk. In this state, as shown in FIG. 22(b)-(1), signals A and B input to a phase comparator are not synchronized with each other. When the rotational speed of the disk gradually increases, as shown in FIG. 22(b)-(2), the PLL is suddenly locked. Since the oscillation frequency of the VCO is lower than the reference clock frequency, a steady-state error remains between the signals A and B. When the rotational speed further increases, as shown in FIG. 22(b)-(3), the frequency of the clock component of the reproduced signal coincides with the frequency of the VCO, and the phase error between the signals A and B becomes zero. When the rotational speed further increases, as shown in FIG. 22(b)-(4), the phase error between the signals A and B increases although the lock is maintained and, at last, the lock is released as shown in FIG. 22(b)-(5). In the unlocked state, the frequency of the signal B becomes a self-oscillation frequency of the VCO.
In the state where the PLL is locked, the PLL generates a clock having a frequency equal to the frequency of the reference clock which is synchronized with the reproduction signal. The CLV control is to make the frequency of the clock output from the PLL equal to and synchronous with the frequency of the reference clock. Therefore, for the CLV control, it is necessary to rotate a motor for rotating the disk, until the state where the PLL can be locked is reached. For this purpose, a rough servo control mentioned below is employed. That is, since a maximum value (T.sub.max) and a minimum value (T.sub.min) of the lengths of signal pits written in the disk or the lengths of spaces between the pits are determined according to the modulation rule, after a signal read from the disk is digitized, the pulse lengths of the signal are measured, and a maximum length or a minimum length is detected from the measured pulse lengths. The rough servo control is to control the spindle motor so that the maximum length or the minimum length becomes a prescribed length.
When the rotation of the disk is controlled on the basis of the information from the reproduction signal RF, even though the tracking servo is operated, off tracking sometimes occurs if the tracking servo is moved due to unwanted disturbance applied to the apparatus, for example, vibration or distortion of the disk surface. When the rotation of the disk is controlled according to the minimum or maximum time length (pulse length) obtained from the reproduction signal as described above, if such off tracking occurs, the minimum time length T.sub.min or the maximum time length T.sub.max is detected incorrectly, and the rotational variation unfavorably increases although the rotation is controlled.
This problems is solved in a disk rotation control apparatus disclosed in Japanese Published Patent Application No. Hei. 5-109182, which apparatus is equipped with means for detecting a tracking error. When a tracking error exceeding a prescribed value is detected, control of a spindle motor on the basis of a reproduction signal RF is suppressed so that the tracking error is not recognized, whereby runaway servo control due to disturbance is avoided.
However, the prior art disk rotation control apparatus has the following drawbacks.
First, when the rotation of the disk is decelerated to stop the rotation by applying a current in the opposite direction to that for the normal rotation, to the spindle motor, the following problem occurs.
When a plurality of disks of different kinds, for example, different diameters or masses, which are rotated by spindle motors, are decelerated to stop the rotation under the same condition, because the disks of different kinds have different inertial moments, some disks are reversely rotated due to over-deceleration and some disks are not completely stopped, while others are normally stopped.
Further, when rotation of a disk is controlled by the CLV method, the rotational speed of the disk varies according to the position of a light beam emitted from the optical pick-up along the radical direction of the disk. Therefore, in a disk rotation control apparatus performing the CLV control, when the rotation of the disk is decelerated to stop the disk from different rotating states of the disk with different rotational speeds under the same condition, whether the disk stops accurately or not depends on the rotational speed of the disk at the start of the operation to stop the disk.
Alternatively, there is a general method for stopping rotation of a disk, wherein the rotation of the disk is decelerated by controlling a spindle motor and, when the rotational speed of the disk reaches a prescribed speed, the deceleration of the rotation is stopped so that the disk stops spontaneously. However, also in this method, when the CLV control is employed, whether the disk stops accurately or not depends on the rotational speed of the disk at the start of the operation to stop the disk, and reverse rotation of the disk occurs sometimes.
Furthermore, a frequency generator is considered as means for measuring the rotational speed of the disk. Although a frequency generator is an effective means for detecting rotational information at the acceleration of the rotation, since the period of a pulse signal output from the frequency generator becomes long at the deceleration, the pulse signal is adversely affected by noise, whereby false detection of rotational information easily occurs. As a result, the disk rotates reversely due to over-deceleration.
By the way, in a disk rotation control apparatus as mentioned above, when the tracking servo is operated, there are two modes for the rough servo control, i.e., a servo control by T.sub.max and a servo control by T.sub.min, and a CLV servo mode for the fine servo control. In the T.sub.min servo control, the minimum time length T.sub.min of the reproduction signal is detected and the disk rotation is controlled according to the minimum time length T.sub.min. In the T.sub.max servo control, the maximum time length T.sub.max of the reproduction signal is detected and the disk rotation is controlled according to the maximum time length T.sub.max. However, when the tracking servo is not operated, in both the T.sub.min detection and the T.sub.max detection, values different from correct values of minimum and maximum time lengths are detected every time the beam spot crosses the track center. Therefore, the rotational variation of the disk increases although the rotation of the disk is controlled. Further, since false detection of T.sub.min and T.sub.max increases, the rotation control system is saturated if the control loop gain is the same as that for the tracking control. So, the loop gain must be set at a low value. Consequently, it is not possible to increase the control loop gain.
When the T.sub.min servo control is performed, since the value obtained by the false detection is smaller than the target value, the rotation control apparatus incorrectly recognizes that the disk is rotated at a rotational speed higher than the target rotational speed. Therefore, in the worst case, the rotation of the disk is stopped.
On the other hand, when the T.sub.max servo control is performed, since the value obtained by the false detection is larger than the target value, the rotation control apparatus recognizes that the disk rotates at a rotational speed lower than the target rotational speed. This results in, in the worst case, runaway operation of the disk rotation control system.
In both cases where the tracking servo is not operated and where it is operated, when rotation of a disk is controlled on the basis of the minimum time length T.sub.min or the maximum time length T.sub.max obtained from the reproduction signal, if the focus servo is moved due to disturbance applied to the apparatus and the light beam is defocused, false detection of T.sub.min or T.sub.max occurs, whereby the rotational variation increases unfavorably even though the rotation is controlled.
When the tracking servo is not operated, off tracking, i.e., deviation of the beam spot from the center of the track on the disk, occurs. However, even when the tracking servo is operated, off tracking occurs if the tracking servo is moved by disturbance applied to the apparatus. When the rotation of the disk is controlled on the basis of the minimum time length T.sub.min or the maximum time length T.sub.max obtained from the reproduction signal, such off-tracking causes false detection of T.sub.min or T.sub.max, whereby the rotational variation increases though the rotation is controlled.
Furthermore, in the conventional disk player equipped with the servo systems as mentioned above, when a recording surface of a disk to be reproduced has an abnormal portion, such as a defect or a flaw, and a beam spot for reading information traces a track in the abnormal portion, an error signal in the servo system is disturbed, whereby the disk player is in danger of malfunction, such as focus servo jumping or tracking servo jumping. In order to avoid the malfunction, when the abnormal portion on the recording surface is detected, the loop gain in the servo system is changed, or the servo control is performed while holding an error value just before the detection of the abnormal portion, or the servo loop is opened. Thereby, stable focus servo control or tracking servo control is performed without a risk of focus servo jumping or tracking servo jumping due to a defect or a flaw on the recording surface of the disk. As a result, the track follow-up ability of the focus servo or the tracking servo is improved. However, such an abnormal portion on the recording surface causes an omission of the reproduction signal, whereby the lock of the PLL is released, resulting in malfunction of the CLV control system or malfunction of the rough servo system due to false detection of T.sub.max or T.sub.min.
Furthermore, the characteristics of the disk rotation control system vary according to the diameter or the mass of the disk. For example, when a 12 cm CD disk and an 8 cm CD disk are compared, since the inertial moment of the 8 cm disk at the rotation is smaller than that of the 12 cm disk, the loop gain intersection of the disk rotation control system for the 8 cm disk is higher than that for the 12 cm disk. FIG. 23 shows T.sub.min control loop characteristics for the 12 cm disk, and the measurement is performed on the innermost circumference of the disk. FIG. 24 shows T.sub.min control loop characteristics for the 8 cm disk, and the measurement is performed on the innermost circumference of the disk. For the 8 cm disk, in order to obtain a gain intersection at a frequency approximately equal to that of the 12 cm disk, i.e., 2.291 Hz, the gain of the 8 cm disk is set at 1/3.2 of the gain of the 12 cm disk. Therefore, it is found from FIGS. 23 and 24 that the loop gain of the 8 cm disk is 3.2 times as high as the loop gain of the 12 cm disk. In this case, even when the loop characteristics are stable for the 12 cm disk, stable loop characteristics cannot be secured for the 8 cm disk.